Model 8901B Service
Service
Sheet
BDI
BLOCK
0
Overall
Instrument
PRINCIPLES
OF
OPERATION
NOTE
The following discussions cover the principles of operation of the Modu-
lation Analyzer. Each discussion is based on and referenced to a service
sheet. An introductory-level discussion
of
the principles
of
operation can
be found in Section
1
of the
Operation and Calibration Manual
under the
title,
Principles of Operation
for
Simplified Block Diagram.
General
The Modulation Analyzer is a general-purpose, tuned, signal-measuring instrument. The Overall Block
Diagram breaks up the instrument’s functions
so
they correspond to those of a receiver: RF Block,
IF
Block, and Audio Block. The Digital Block (using the receiver analogy) roughly corresponds to
the human operator. Each
of
the functional blocks
is
shown in greater detail in Service Sheets BD2
through BD5.
Sensor Module
During normal operation. the RF input signal
is
applied to an external Sensor Module such as the HP
11722A. The Input Switch in the Sensor Module routes the RF signal to the Power Sensor when the
RF Power measurement mode is selected. The Power Sensor converts the RF input signal to a chopped
dc voltage which
is
proportional to the average power level
of
the signal.
For
other measurement modes,
the switch routes the input signal
to
the INPUT connector of the Modulation Analyzer. The Sensor
Module
also
contains a resistor which provides a means for the Modulation Analyzer to identify the
type of Power Sensor in the Sensor Module.
Power Meter
The Power Sensor and Power Meter amplify the dc voltage from the Power Sensor’s transducer
so
the
voltage can be measured by the Voltmeter in the Audio Block. To minimize the effect of dc
drift
in the
amplifiers (which may even exceed the dc voltage from the transducer itself), the transducer’s output
is converted to an ac voltage by
a
chopper in the Power Sensor. The chopper drive comes from the
220 Hz Multivibrator. The resultant ac signal is ac coupled to the narrow-band Input Amplifier, and
the gain of this amplifier is autoranged by the Controller. The amplified ac signal
is
converted back
to dc by the Synchronous Detector, which full-wave rectifies the ac signal by rechopping it in phase.
Noise
on
the resultant dc signal is filtered by the Noise Filters to stabilize the displayed reading. The
heaviest filtering
is
done on the most sensitive ranges and results in an increase in the measurement
response time. Before an RF power measurement is initiated, the switch at the output of the Power
Meter connects the Sensor Identifier Resistor circuit to the Voltmeter. The voltage across the resistor
indicates the type of Power Sensor present.
The Zeroing DAC
is
set by the Controller
to
cancel any dc offset of the Power Sensor when no input
is
applied. In the zeroing sequence, the RF Input to the Power Sensor
is
switched out,
a
power reading
is taken, and the Zeroing DAC
is
automatically adjusted for a zero reading. (In actual implementation,
the Zeroing DAC
is
adjusted
for
a pre-determined offset voltage reading. The offset
is
subtracted out
in subsequent power measurements by the Controller. The Voltmeter then operates in its most linear
region.)
Service Sheet
BD1
8D-1
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